Tolstykh was one of the most recognizable faces of the armed conflict in Ukraine

Mikhail Tolstykh, known by his nom de guerre Givi, was a prominent separatist commander. He died Wednesday in an explosion in his office in eastern Ukraine. In this photo, he speaks to reporters near Donetsk's Sergey Prokofiev international airport on Oct. 9, 2014. (John MacDougall/AFP/Getty Images)

A prominent rebel leader in eastern Ukraine has been killed in an explosion in his office, his associates said on Wednesday.

The rebels' Donetsk News Agency said Mikhail Tolstykh, better known under his nom de guerre Givi, died early Wednesday morning in what it described as a terrorist attack. The agency said 35-year old Tolstykh was killed by a rocket fired from a portable launcher into his office.

Russian state television showed pictures of firefighters putting out flames in the building where Toltsykh's headquarters is believed to be. The footage from the scene showed several rooms in the building gutted from an apparent explosion.

Givi casts his ballot during supreme council and presidential elections on the frontline near Donetsk airport in eastern Ukraine on Nov. 2, 2014. (Dmitry Lovetsky/Associated Press)

Tolstykh's death follows the assassination of his close associate Arsen Pavlov, also known as Motorola, last year, as well as other high-profile warlords.

Yuri Tandit, an adviser to the chairman of the Ukrainian Security Service in Kyiv, said on the 112 television channel that his agency was looking into the reports.

9,800 killed since 2014

Tolstykh was one of the most recognizable faces in the conflict between Ukrainian government troops and Russia-backed rebels which has claimed more than 9,800 lives since it began in 2014.

Killings of high-profile commanders in Ukraine's Donbass began in May 2015 with the bombing of the charismatic Alexei Mozgovoi. Rank-and-file separatists and local residents reported an increased Russian influence in the area in summer 2015 as Moscow apparently tried to rein in the warlords, some of whom seemingly got out of hand with murder and violence targeting civilians.

Givi had recently taken part in the battle for Avdiivka, just outside Donetsk. Here, tanks are seen in the industrial town on Feb. 2. (Gleb Garanich/Reuters)

The very existence of unruly commanders like Givi bolstered the Ukrainian government's long-standing refusal to negotiate with what it regarded as terrorists. Givi and other warlords who have been killed in the past two years have publicly assaulted prisoners of war and been engaged in what can be classified as war crimes.

While the unruly commanders were dying in car bombings, the leadership of the rebel-controlled parts of the Donetsk and Luhansk regions came to be dominated by bureaucrats with ties to ousted pro-Russian President Viktor Yanukovych, a Donbass native, rather than the commanders who led the uprising.

Unlike the assassinated warlords, the Donetsk bureaucrats are seen as less extreme and more inclined to bargain with Kyiv.

Dmitry Peskov, spokesman for Russian President Vladimir Putin, described Tolstykh's death as an attempt to "destabilize the situation" in eastern Ukraine after flare-up of hostilities last week killed more than 33 people.

Peskov denied any Russian involvement in the warlord's death, calling it impossible.